Electrical-Optical (E-O) interfaces are used in high-speed communication systems to convert an electrical signal into an optical signal. Typically, the E-O interface core includes a modulator configured to generate modulated light power as a function of an electrical signal, and a driver which receives, at an input, the electrical signal from preceeding electronic stages and drives the modulator with voltage and current of sufficient magnitudes.
The driver may be increasingly important in high data rate (DR) applications, since relatively large output voltage levels with steep rising and falling edges may be desired for proper operation of the modulator and with limited jitter so as to not degrade the transmitted bit stream. At the same time, it may be desirable that the driver be designed to reduce its power consumption since it may contribute to the overall power budget of a typical optical link.
In the specific case of Mach Zehnder Modulators (MZMs), due to the geometrical size of the interferometer used to build the MZM, for high speed operation, the driver is often split into several stages. Each stage may drive a portion of the MZM. Thus, a distributed modulating structure may be formed. In this case, it may be desirable that the driver stages allow their intrinsic delays to be programmable for proper operation of the distributed architecture by equalizing the optical delay of the light propagating within the MZM optical guides, with the delay of the electrical signal propagating through the distributed driver stages.
The load to be driven generally includes a plurality of reverse-biased junctions (e.g. MZMs), with an equivalent circuit including a capacitive load. Typical high speed driver implementations at data rates of tens of GHz rely on relatively expensive materials, such as Gallium Arsenide, Indium Phosphide, Indium Gallium Arsenide, or expensive technological platforms, such as, Silicon-On-Insulator, and make use of relatively large supply voltages (e.g. 5V). Recently, silicon CMOS and BiCMOS implementations are being proposed, in particular, for silicon photonics applications. Common high speed driver architectures include travelling wave amplifiers using cascoded cells, as described in “Design of an opto-electronic modulator driver amplifier for 40-Gb/s data rate systems,” Long, A., Buck, J., and Powell, R., Journal of Lightwave Technology, Volume: 20, 2002 Page(s) 2015-2021, and “Ultra-low voltage substrate-removed mach-zehnder intensity modulators with integrated electrical drivers,” Dogru, S., JaeHyuk Shin, and Dagli, N., LEOS Annual Meeting Conference Proceedings, 2009, LEOS '09, IEEE, Page(s) 656-657, or differential pairs, as described in “A Fully Integrated 20-Gb/s Optoelectronic Transceiver Implemented in a Standard 13-CMOS SOI Technology,” Analui, B., Guckenberger, D., Kucharski, D., and Narasimha, A., IEEE Journal of Solid-State Circuits, Volume 41, 2006, Page(s) 2945-2955 and “Power Efficiency Comparisons of Interchip Optical Interconnect Architectures,” Palaniappan, A. and Palermo, S., IEEE Transactions on Circuits and Systems II, Express Briefs, Volume 57, (2010), Page(s) 343-347.
MOS transistors with thin gate oxide formed with the most advanced technology nodes may lend themselves to the realization of high speed drivers, for example, as described in “A 40-Gb/s Optical Transceiver Front-End in 45 nm SOI CMOS,” Joohwa Kim and Buckwalter, J. F., IEEE Journal of Solid-State Circuits, Vol. 47, 2012, Page(s) 615-626, and U.S. Pat. Nos. 7,899,276, 7,515,775, 7,450,787 to Kucharski et al. and U.S. Pat. No. 7,039,258 to Gunn, III et al. However, due to the oxide thinness, they may be subject to Safe Operating Area (SOA) issues when used in drivers with a relatively large output voltage (e.g. >2V), as may be with typical modulator implementations (e.g. MZMs).